The long-term objective of this research is to understand how animals and humans perceive their sensory worlds. In the study of a sensory system, it is essential to know what the system is designed for. An animal's sensory systems must be designed for perception of sensory signals that are important for its survival and reproduction. The barn owl uses hearing to localize prey in dark nights. Its auditory system should therefore be designed for sound localization. This expectation was correct and led to important findings about how the owl's brain processes complex sounds for prey localization. Other sensory systems and animals appear to employ the same general methods of signal processing used by the owl. The simple auditory system of the owl can serve as a model for the study of other sensory systems including those of humans. The owl 's auditory system analyzes complex sounds in several steps. Some of the key steps have been studied in depth and others remain to be explored further and constitute the specific aims of this research. The owl uses differences in time of arrival of sound at the two ears for localizing sound in the horizontal plane and differences in loudness between the ears for the vertical plane. The owl's auditory system processes time and loudness in different neural pathways. Whereas the time pathway has been extensively studied, much work needs to be done on the loudness pathway. The methods of study are to look for the flow of information about differences in loudness by observing the electrical activities of single nerve cells and tracing the connections between them.